Silver brazed titanium composite

ABSTRACT

A method is disclosed for the silver brazing of titanium workpieces to avoid the formation of the brittle silver-titanium intermetallic compound. In a preferred embodiment a thin film of chromium is vapor deposited on titanium surfaces to be brazed. Silver brazing is then accomplished by conventional methods, the chromium layer permitting the interdiffusion of silver and titanium but preventing the formation of a brittle intermetallic compound. The brazed product has a ductile alloy bond between the titanium sections comprising titanium, chromium and silver.

United States Patent [54] SILVER BRAZED TITANIUM COMPOSITE 3 Claims, 8 Drawing Figs.

52 u.s.c1. 29/198, 29/199 [51] 1 1:.c1 B32b /00 50] FieldofSearch 29/198, 194,199

[56] References Cited UNITED STATES PATENTS 2,798,843 7/1957 Slomin 29/ l 98 2,674,542 4/1954 Alexander 29/198 2,908,966 10/1959 Wagner 29/194 2,906,008 9/1959 Boegehold 29/198 Primary Examiner-I-lyland Bizot Attorneys-Peter P. Kozak and George A. Grove ABSTRACT: A method is disclosed for the silver brazing of titanium workpieces to avoid the formation of the brittle silver-titanium intermetallic compound. In a preferred embodiment a thin film of chromium is vapor deposited on titanium surfaces to be brazed. Silver brazing is then accomplished by conventional methods, the chromium layer permitting the interdiifusion of silver and titanium but preventing the formation of a brittle intermetallic compound. The brazed product has a ductile alloy bond between the titanium sections comprising titanium, chromium and silver.

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g 5.0 CHROMIUM x as DISTANCE FROM CENTER OF BRAZE-MICRONS PATENTEI] S, EP21 Ian 3,607,152

' sum 1 or 3 PRIOR ART INVENTOR ATTORNEY PATENTEU SEPZI IHYI sum 3 0F 3 SILVER VANADIUM TITANIUM ALUMINUM [L'LIIL'L o m w w w w w 5.6252. m-x

LIIIF'LIILIFIL 0000 8642 DISTANCE FROM CENTER PRIOR ART BRAZE WlTH SILVER ALLOY CLEAN TITANIUM SU RFACE ATTORNEY SILVER BRAZED TITANIUM COMPOSITE This is a division of SN. 550,994 filed May 18, 1966, now US Pat. No. 3,466,737, issued Sept. 16, 1969.

The invention herein described was made in the course of work under a contract or subcontract thereunder with the Department of Defense.

This invention relates to the fabrication of titanium and titanium alloy components or sections into useful articles of manufacture. More particularly, it relates to an improved brazed joint between titanium components and to an improved method of brazing such components. As used herein titanium refers to both titanium and titanium alloy components.

In the prior art titanium components have been brazed using suitable silver alloy brazing materials. However, it is known that molten silver will react with titanium to form a brittle intermetallic compound which weakens the joint and impairs its fatigue resistance. This has meant in general that a brazing cycle could not be used in which the silver alloy remained in the molten state for more than about minutes, if the brazed joint was designed to withstand relatively severe stresses or to be subjected to cyclic loads. For this reason engineers have not been able to take advantage of the generally superior mechanical and chemical properties of titanium in many engineering applications. When massive titanium-based components were silver brazed, the heat content of the components was such that the silver remained in the molten state for a period sufficient for the formation of the brittle silvertitanium compounds at the interface of the braze and the titanium member. Therefore, the prior state of the art has been lacking in the ability to silver braze titanium by using conventional brazing heating cycles to achieve mechanical properties which take full advantage of titaniums high strength-to-weight ratio necessary in efficient, relatively lightweight, large structures.

It is an object of this invention to provide a strong, ductile, fatigue-resistant silver alloy brazed joint between titanium sections in an article of manufacture.

It is a more specific object of this invention to provide a strong silver alloy brazed bond between large titanium sections when a brazing cycle is required in which molten silver is in contact with a titanium or titanium alloy member for substantially longer than about I5minutes.

It is a further object of this invention to provide a silverbrazed titanium joint wherein the critical portion of the joint, the interfacial layer between a titanium section and the silver alloy braze, contains chromium. This interfacial portion of a joint is thus comprised of a ductile alloy comprised of chromium, silver, and titanium; no brittle intermetallic compound being formed.

It is another object of this invention to provide an improved method of brazing titanium or titanium alloy sections with a silver-brazing alloy in which in the preferred embodiment chromium is first applied to the mating titanium surfaces to prevent the formation of brittle intermetallic compounds of titanium and silver.

In accordance with my invention, these and other objects are accomplished in a preferred embodiment by applying, as for example by depositing, plating or the like, a thin film or layer of chromium to each of the titanium surfaces which are to come in contact with the molten silver-brazing alloy. Preferably, the titanium surfaces are clean so as not to interfere with the chromium deposition or the flow of the silver alloy during the subsequent brazing step. Sufficient chromium is applied to alloy with the titanium and silver during the braze cycle. In general, a chromium layer about 0.00005 to 0.001 inch in thickness is preferred. When a chromium film of suitable thickness has been applied, the parts are then brazed by techniques well known in the art. The metallurgical bond is thus comprised of the ductile alloy comprised of silver, chromium, and titanium and no brittle intermetallic compound is formed.

Other objects of my invention will become apparent to.

those skilled in the art in view of the following detailed description thereof, reference being made to the enclosed figures in which:

FIG. 1 is a photomicrograph at 500X showing a brazed joint between titanium and silver-brazing alloy, chromium having been plated on the titanium prior to brazing in accordance with my invention;

FIG. 2 consists of two photomicrographs, at 2,500X (FIG. 2A) and 10,000X (FIG. 28) respectively, of the interface between titanium and silver-brazing alloy, chromium having been plated on the titanium prior to brazing;

FIG. 3 includes two photomicrographs (FIG. 3A at 2,500X and FIG. SE at 10,000X) similar to FIG. 2 of a silver-brazed titanium joint in which no chromium was used in accordance with prior art techniques;

FIG. 4 is a graph illustrating the distribution of elements across a titanium joint brazed with silver alloy, a chromium layer having been deposited on titanium components prior to brazing;

FIG. 5 is a graph illustrating the distribution of elements across a titanium joint brazed with silver alloy in accordance with the prior art;

FIG. 6 is a flow diagram illustrating a preferred embodiment of the process.

I have discovered that chromium may advantageously be applied to titanium surfaces in extremely thin films to effectively prevent the formation of the brittle intermetallic compound during silver brazing cycles of practical duration. Some prior art techniques have attempted to inhibit the silver-titanium reaction by providing a metallic barrier layer over the titanium sections. However, these barriers (e.g., of cobalt or iron) have not been effective because the molten silver dissolves them or diffuses through them during the brazing operation to react with the titanium. However, in accordance with my invention, the chromium layer does not act as a barrier between the silver and titanium. It does not prevent the contact of the molten silver with the titanium. Surprisingly, however, it does prevent the formation of the intermetallic compounds. During the brazing cycle the molten silver braze alloy contacts, mixes with, or diffuses into the chromium deposit and the titanium substrate to form a chemical alloy bond comprised of at least silver, titanium, and chromium.

In accordance with the preferred embodiment of my invention chromium is applied by depositing, plating or the like in a thin film or layer to the clean titanium or titanium alloy surfaces, which are to be brazed, by any suitable deposition, coating, or plating technique. Vapor deposition of the chromium, a technique well known in the art, is particularly suitable because an extremely thin film of relatively uniform thickness may be obtained. The deposit is preferably about 0.00005 inch to 0.001 inch in thickness. Subsequently, the chromium coated titanium components may be brazed by techniques which are now being used in the art. Both the vapor deposition step and the brazing step will be described in more detail below.

FIGS. 1-3 illustrate the effectiveness of my discovery and will also serve as a means for defining and characterizing the silver-brazed titanium joint which comprises a pan of my invention. In FIG. I the photomicrograph (at 500X) shows the brazed jointbetween the titanium sections 10 comprised, by weight, of 6 percent aluminum, 4 percent vanadium and the balance substantially all titanium. The brazing alloy 12 was comprised of percent silver and 5 percent aluminum by weight. Chromium vapor had been deposited on the titanium surfaces prior to brazing. The titanium surfaces were in contact with molten silver for about 40 minutes during the brazing cycle. The photograph indicates the absence of brittle intermetallic compound from the interfacial area 14 between the silver alloy 12 and the titanium members 10.

The two photomicrographs comprising FIG. 2 also show the interface region 14 between the same titanium alloy and brazing alloy but at a higher magnification. Chromium was vapor deposited on the titanium prior to brazing. FIG. 2a is at 2,500X and FIG. 2b is at 10,000X. No intermetallic compound is prevent in the interfacial area 14 which comprises the chemical or metallurgical bond between the silver braze and titanium members.

However, FIG. 3 shows a brazed joint between the same titanium and brazing alloys in which no chromium coating was employed. The presence of the brittle intermetallic compound in he interface layer 14 is readily apparent. FIG. 3a is at 2,500X and FIG. 3b is at 10,000X. FIGS. 2 and 3 were electron photomicrographs.

In the case of silver-brazing titanium sections involving the specific titanium and silver alloys defined above, the reaction product creating the metallurgical bond is an alloy comprised of silver, vanadium, titanium, aluminum, and chromium in the relative amounts shown in FIG. 4. FIG. 4 represents an electron probe analysis of the interface portions 14 between the silver braze section 12 and the titanium sections 10. The results indicate that the bonding alloy interface 14 between the titanium and silver sections is about microns in thickness. I have found that it is possible to form a bond interface of mutually alloyed elements of up to at least about 0.002 inch in thickness. As would be expected, and as is shown in FIG. 4, the composition of the interfacial alloy is not constant throughout its thickness due to diffusion of the various elements. Silver attains a maximum concentration in the interfacial portion adjacent the silver-brazing alloy and decreases to a minimum value in the interfacial portion adjacent the titanium bonding alloy member. Conversely the titanium concentration in the interfacial alloy area is a maximum value adjacent the titanium member and decreases in the direction of the silver-brazing section. It is to be noted however that the chromium concentration attains its maximum value well within the interface layer and is of lower level at the titanium and silver surfaces. It is apparent that the presence of chromium does not prevent the alloying of the silver and titanium.

FIG. 5 is an electron probe analysis of a brazed joint of the same titanium alloy and silver-brazing alloy except that no chromium has been applied to the titanium surfaces prior to the brazing. It is noted that an alloy interfacial layer is formed which is similar in silver and titanium concentrations to that shown in FIG. 4.

Thus, it is seen that the presence of chromium at the braze interface gives a heretofore unobtained and unexpected result during the brazing operation. While contact between the silver and titanium and other alloying constituents is not impeded, there is, nevertheless, no brittle intermetallic compound formed.

The effectiveness of the presence of chromium in the interfacial area between the silver braze and the titanium component is also manifested in the physical properties of the article.

Tests conducted with the above-defined titanium and silver alloys show higher shear strength for the chromium treated titanium when the brazing cycle exceeds about 5 minutes at [550 to l,650 F. In general the longer the period in which molten silver is in contact with the titanium the greater the difference in shear strength between the chromium-containing joint and brazes accomplished in accordance with prior art techniques. As noted above a practical brazing cycle for many applications requires that the silver braze be in the molten state for about 40 minutes or more.

In addition, fatigue tests have shown a major improvement in the fatigue endurance level of chromium treated long cycle brazes over untreated long cycle brazed joints. It is clear that with the practice of my invention full and practical braze cycles can be utilized in the forming of high strength large titanium structures.

While the techniques of applying chromium to a substrate by vapor deposition and of silver-brazing titanium sections are in general known in the art, a specific example of a preferred embodiment will nevertheless more completely illustrate my invention. In general, it is preferable that the titanium details, or surfaces to be brazed, be clean prior to deposition of the chromium. When necessary titanium surfaces may be cleaned,

for example, by wet vapor blasting all areas intended for brazing using a 325 grit silica flour lightly scrubbed in clean water. The surfaces may then be dried with forced air. Subsequently, it may be advantageous to rinse the surfaces in acetone and redry using forced air.

Titanium sections to be brazed may then be placed in a chamber suitable for vacuum deposition of chromium. It is preferable in accordance with my invention that pure chromium chips (99.999 percent chromium) be used. Surface portions of the titanium section which are not to be brazed may be masked if desired to prevent deposition in these areas. The vaporizer source may be filled with suflicient chromium chips to provide a film on the titanium surface, preferably from about 0.00005 inch to 0.00l inch in thickness. These are approximate figures. The lower value is approximately the minimum thickness of chromium that I have been able to obtain by vapor deposition. The higher value is approximately the maximum amount of chromium which can readily be incorporated or alloyed at the silver braze-titanium substrate interface into the metallurgical bond during a normal brazing cycle. A thicker deposit of chromium, while preventing the formation of the intermetallic compound, may in some situations weaken the brazed joint. The preferred position of the titanium workpiece with respect to the chromium source will, of course, have to be determined for each particular vapor deposition apparatus and each specific workpiece configuration. In some cases multiple chromium sources and/or workpiece traversing may be required. The vapor deposition chamber preferably should be evacuated to an absolute pressure of approximately 5 XlO millimeters of mercury. The chromium vaporizer source is heated, as for example by electrical resistance heating elements, until sufficient chromium has evaporated and deposited on the workpiece. The chromium plated titanium sections may then be brazed.

The titanium sections are placed in a suitable brazing fixture so that they may be maintained in a proper position wherein intimate contact is maintained throughout the brazing cycle. As is known he strength of the brazed joint depends greatly on the clearance between the sections before brazing. It is common practice whenever possible to provide clearances of about 0.001 inch to 0.008 inch. In general, clearances of 0.001 inch to about 0.005 inch are preferred. The molten silver-brazing alloy will readily creep upward or horizontally on the surface of such a joint by capillary action or downward under the attraction of gravity. The fixtures and all accessories should be clean. Silver-brazing alloy my be located between or adjacent the components to be joined and the fixture halves secured together to supply necessary pressure to obtain intimate contact at the areas of the braze. Then upon heating to temperatures above the melting point of the alloy molten braze material flows between the closely spaced titanium components. The brazing cycle preferably is performed in a suitable furnace adapted to maintain an inert atmosphere. The brazing of titanium components in argon atmosphere is preferred. In accordance with my invention the brazing temperature may be as high as 1,550" l,650 F. or higher and molten silver-brazing alloy may be allowed to remain in contact with the titanium components for periods as long as 40 minutes or more as is required by the normal heating, brazing, and cooling cycles for massive sections.

As soon as cooling is started the molten silver-brazing alloy solidifies, at least a portion thereof having now alloyed with the titanium and the chromium, to form a strong but ductile and fatigue-resistant bond which includes no brittle intermetallic compound. Subsequent to brazing, the fabricated titanium assembly may be annealed in accordance with prior techniques.

Although the electron microprobe analyses referred to above indicate relative amounts of the various constituents in the interface layer, it is difficult to precisely state the composition limits. However, it is known that in normal brazing cycles there is time only for a limited amount of diffusion of chromium away from the joint and that therefore a layer of chromium preferred embodiment when the maximum clearance between titanium sections is less than about 0.005 inch, the overall composition of the interfacial layer is thus practically limited by a material balance of the chromium and silver-brazing alloy which have been used.

In the description of my invention, reference was made to a titanium-bases alloy containing 6 percent aluminum and 4 percent vanadium by weight, and to a 95 percent silver, 5 percent aluminum by weight brazing alloy. It is to be understood, however, that my invention comprehends any brazing environment or composition in which molten silver can react with titanium to form a brittle intermetallic compound. Silverbrazing alloys comprising, for example, by weight, 50 percent to 95 percent silver, to percent aluminum, 0 to 40 percent copper, 0 to 5 percent zinc, 0 to 1 percent nickel and 0 to 0.2 percent lithium are commercially available. Any of these alloys contain sufiicient silver to react with titanium during an extended brazing cycle. Similarly, titanium-based alloys other than that defined above are attacked by silver unless my invention is practiced.

It is also to be understood that means other than vapor deposition might be used to incorporated chromium into the brazed joint. For example, chromium might be alloyed with the silver braze material. Alternatively, small amounts of chromium might be plated, sprayed or otherwise incorporated onto or into the titanium surfaces, which are to be joined, by techniques other than vapor deposition. Workers skilled in the art would also recognize that laminated brazing materials are used, and in this case a laminated brazing material comprised of chromium and silver alloy might find application.

Thus, while this invention has been described in terms of the specific embodiment, it will be recognized that other forms might readily be adopted by those skilled in the art; and therefore the scope of my invention is considered limited only by the following claims.

I claim:

1. A brazed joint between at least two titanium or titanium alloy sections comprised of a major proportion of silver in the center portion of said joint; a major proportion of titanium in the facing surfaces of each of said titanium sections; and two ductile alloy portions comprised of chromium, silver, and titanium adjacent each of said titanium sections, the concentration of silver in said ductile, alloy portions decreasing in the direction toward said titanium sections, the concentration of titanium in said ductile alloy portions decreasing in the direction toward the center of said joint, and the concentration of chromium being of its maximum value near the center of said ductile alloy portion.

2. An article of manufacture comprising at least two titanium or titanium alloy sections brazed together with silverbased braze alloy in a ductile, fatigue resistant difiusion bond, the said brazed bond between said titanium sections being characterized by a silver alloy layer disposed between the facing surfaces of said titanium sections and ductile interfacial layers between said silver alloy layer and each of said titanium sections, said interfacial layers comprising chromium, silver and titanium, the concentration of silver in a said interfacial layer being a maximum adjacent said silver braze alloy layer and decreasing in a direction toward the adjacent said titanium section, the concentration of titanium in a said interfacial layer being a maximum at a point adjacent a said titanium section and decreasing in the direction toward said silver braze alloy layer, and the concentration of chromium in a said interfacial layer being a maximum value near the center of said interfacial layer.

3. A brazed article as recited in claim 2 wherein said silver braze alloy initially comprises about 50 percent to percent by weight silver. 

2. An article of manufacture comprising at least two titanium or titanium alloy sections brazed together with silver-based braze alloy in a ductile, fatigue resistant diffusion bond, the said brazed bond between said titanium sections being characterized by a silver alloy layer disposed between the facing surfaces of said titanium sections and ductile interfacial layers between said silver alloy layer and each of said titanium sections, said interfacial layers comprising chromium, silver and titanium, the concentration of silver in a said interfacial layer being a maximum adjacent said silver braze alloy layer and decreasing in a direction toward the adjacent said titanium section, the concentration of titanium in a said interfacial layer being a maximum at a point adjacent a said titanium section and decreasing in the direction toward said silver braze alloy layer, and the concentration of chromium in a said interfacial layer being a maximum value near the center of said interfacial layer.
 3. A brazed article as recited in claim 2 wherein said silver braze alloy initially comprises about 50 percent to 95 percent by weight silver. 